Phase and frequency modulation system



Feb, 27 E951 L. T. BlRD 2,543,222

PHASE AND FREQUENCY MODULATON SYSTEM Filed Oct. 11, 1945 2 She'S-Sheet lrif/f1 Feba 27, 1951 L. T. BIRD PHASE AND FREQUENCY MODULATION SYSTEM 2shets-sheet 2 Filed Oct. ll, 1945 Ff .f

yf By Patented Feb. 27, 1951 Ni'lED STATES FATE? PHE PHASE AND FREQUENCYMODULATION SYSTEM 14 Claims.

This invention relates to improvements in both phase modulation andfrequency modulation transmission systems.

ln systems at present in use, phase modulation is eiected by theaddition of two radio frequency currents having a fixed phase differenceof 9G", the phase of the combination being shifted in response to themodulating E. M. F. by applying linear amplitude modulation to one orboth oi the component currents. The Weakness of such systems is that theangle of phase shift is not directly proportional to the amplitude ofthe modulating E. M. Ir". unless the angle is Very small, and, forfreedom from distortion in transmission of music, it is necessary forthe maximum angle to be limited to much less than 30. This restrictionmaires it necessary to employ many stages of frequency multiplicationafter modulation in order to effect the required phase shift orfrequency swing in the output currents, necessitating complicatedsystems of heterodyning to arrive at the desired unmodulated carrierfrequency when the required phase or frequency shift is achieved. Inaddition, unless special means are adopted to avoid it7 the highfrequency-multiplication ratio imposes severe demands upon the frequencystability oi the initial source of radio frequency current.

It is the object of the present invention to ren move these restrictionsand avoid these difliculties by providing means whereby, to a very closedegree of approximation, the angle of phase shift to any predeterminedlimit is kept strictly proportional to the amplitude of the modulatingThe invention is illustrated in the accompanying drawings in whichFigure l represents the preferred embodiment of the system; and

1Figures 2, li, 5, 5, 7, 8, 9 and 10 illustrate the relationship betweenthe E. M. FXS appearing at various points in the system.

Referring now to Figure 1, I is a source of radio frequency (R. F.)energy such as a master oscillator which is preferably crystalcontrolled. 2 is a variable resistor, and 3 a capacity connected inseries across the output or" source l.

A resistor d, similar to in series with an inductor 5 the reactancc ofwhich is approximately ecualto that of capacity 3 is also connectedacross the source i, and is a potentiometer, hich is also connectedacross said source.

l5 is a source of modulating energy the output oi which is fed inparallel to the primary windings of two transformers il and i8.

I9, 2o, 2l and 22 are rectii'iers, represented as diodes. The secondarywinding of transformer il' is connected at one end to the cathode ofdiode IS, and at the other end to the cathode of diode 22, and twotapping points, situated respectively between each end of said secondarywinding and a central tapping point on said winding, are connectedrespectively to the anodes of diodes le and 2i. The anodes of diodes i9and 22 are connected together and the cathodes of diodes 2S and 2i arealso connected together, the latter common connection being connectedthrough a resistor t? and a further resistor 28 to said central tappingpoint, and the junction point of said resistors 2l and 23 is connectedthrough a source of D. C. potential 3E to a further resistor 29connected to the anodes of diodes I9 and 22.

23, 2d, 25 and 25 are rectiners, also repretransicrmer i8 is connectedat one end to the cathode of diode Z6 and to the anode of diode 23, theother end of said secondary winding being connected to the cathode ofdiode 25 and to the anode of diode Q4. The cathode of .diode 23 isconnected through resistor 3|, sources of D. C. potential 38 and 39, andresistor. 33 to a central tapping point upon said secondary winding, andsented as diodes. llhe secondary winding of the the anode of diode 2t isconnected through re sistor 35, sources of D` C. potential il and lli),and resistor 33 to said tapping point. The cathode of diode zal isconnected through resistor 32 to the junction oi sources 38 and 39, andthe anode of diode 25 is connected through resistor to the junction ofsources lili and lil. An additional secondary winding on the core of thetransformer i 8 is connected at one end to the central tapping point onthe secondary of transformer' IS and is shunted by a resistor 3Q.

E3, I4 and l5 are amplifier tubes, shown for the purpose of thisexplanation as pentodes, bias potential being applied through resistor 8from source B, resistor l0 from source 6I, and resistor I2 from source52 to the control grids of said tubes respectively. Heating current isapplied from sources t3, 6e and 65 to the cathodes of said tubesrespectively. D. C. potential for the screen grid of tube i 3 issupplied by means of resistor El connected across source 5l. Similarly,potential for the screen grid of tube I4 is supplied by means ofresistor t8 and source 69, and for tube i5 by means of resistor 'IQ andsource 'il Fixed potential is applied to the supof said potential beingadjusted by means of resistor l2. The anode of tube I3 is connected toone side of a tuned circuit constituted by variable capacity 43 andinductor 44, the other side of said tuned circuit being connected to thepositive terminal of source 61. Similarly, the anodes of tubes i4 and I5are connected through tuned circuits 45, 47 and 4G, 55 to the positiveterminals of sources 55 and 'II respectively. The cathodes of tubes I3,I4 and I5 are connected through a common lead to source I. A source ofD. C. potential 3'.' is connected between said common lead and thejunction points of resistors 21 and 28, and a further source of D. C.potential 42 is connected between said common lead and the junctionpoint of sources 39, 43 and resistor 33. A variable tapping point onresistor 2 is connected through capacity I I to the control grid of tubeI5, a variable tapping point on resistor 4 is connected through capacity9 to the control grid of tube I4, and a variable tapping point onpotentiometer l is connected through capacity 'l to the control grid oftube I3. The central tapping point on the secondary winding oftransformer I! is connected to the suppressor grid of tube I4, and theend of resistor 3G remote from the junction thereof with the centraltapping point on the secondary winding of transformer I8 is connected tothe suppressor grid of tube I5.

To the inductors 44, 4l and 55 are coupled coils, respectively 45, 48and 5I, and said coils are connected to a network of resistors 52 to 58,which are all of like value.

A conventional balanced bridge circuit is formed by the resistors 52, 53and 54 connected in series and the total resistance represented by I thenetwork made up of resistors 55 to 58 by connecting one end M ofresistor 52 to the junction point R, of resistors 5S and 5l and one endN of resistor 54 to the junction point S of resistors 55 and 58. thejunction point P of resistors 55 and 56, the other end being connectedto junction point Q of resistors 51 and 58. One end of coil 48 isconnected to the junction point T of resistors 53 and 54 and the otherend is connected to the junction of points R and M. Coil 5I is connectedat one end to the point N and at the other end to the junction point Oof resistors 52 and 53. Output is taken from across resistor 51 betweenthe points Q and R to utilization means 59, which may consist offrequency multipliers, filters and power amplifiers as may be requiredand in accordance with well known and established practice.

The operation of the system will now be described:

The output of oscillator I, which operates at a frequency considerablylower than that at which iinal emission from the system takes place,causes an E. M. F. to be developed across the capacity 3, the adjustmentof resistance 2 being set so as to cause the E. M. F. applied to controlgrid of tube I5 through capacity I I to lag that of said output by 45.The value of resistor I2 is high in comparison with the reactance ofcondenser 3. The tapping point on resistor 4 is adjusted so that the E.M. F. applied through capacity 9 to the control grid of tube I4 leadsthat of the output of oscillator I by 45. Resistor I is similar toresistor I2. The potentiometer E is adjusted so that the E. M. F.supplied to the control grid of tube I3 through capacity 'I is in phasewith that of the oscillator I.

The source I6 is of constant gain across the band of modulationfrequencies in the case of.

Coil 45 is connected at one end to J'- phase modulation and inverselyproportional to the frequency in the case of frequency modulation. Theoutput from I5 is fed to transformers I? and I8 which step up themodulating voltage to the values required to operate the diodes I9 to 25in the following manner: Let it be assumed for the purpose ofexplanation that the sense of the secondary winding of transformer Il issuch that, at the instant considered, that end which is connected todiode I9 is positive. In these circumstances the E. M. F. developed bythe secondary will tend to make its cathode positive with respect to itsanode and no current will i'iow through it. Similarly, the anode ofdiode 2I will tend to become negative with respect to its cathode andit, too, will be inoperative. Diode 20, however, will, at the instantconsidered, have its anode rendered positive with respect to its cathodeand this will cause a current to flow through resistors 2 and 23 whichwill set up a potential across resistor 23 the negative end of whichwill be the one nearer the connection to the transformer secondary.Similarly, diode 22 will have a negative E. M. F. applied to its cathodeby the transformer secondary and the potential across resistor 2S.However, the voltage of source 35 is so chosen that it exceeds theseapplied potentials until a certain predetermined value is reached. Forall values of modulating voltage up to this, the potential acrossresistor 28 will be directly proportional to the instantaneous value ofthe modulating E. M. F. When this value is exceeded, current will flowthrough diode 22, resistor 29 and source 36, and return to thetransformer through resistors 23 and 2l. That portion which ows throughresistor 28 will oppose the potential set up across it by current fromdiode 20. The E. M. F. of the transformer secondary and the values ofresistors 2l, 23 and 29 are so chosen that as the modulating E. M. F.increases from this point on. the additional current in resistor 28, dueto the action of diode 22, causes the potential across resistor 28 todecrease with increasing E. M. F. at the same rate as it previouslyincreased, until, eventually, at an E. M. F. corresponding to maximummodulation, the potential across'resistor 28 will have fallen to zero.When the E. M. F. of the secondary is in the reverse sense, the sameresult is obtained, except that now diodes I9 and 2l are active anddiodes 2) and 22 are inactive. Thus the relation between instantaneousaudio E. M. F. and instantaneous potential across resistor 28 will be asshown in Figure 2. Referring again to Figure 1, the potential acrossresistor 28 is applied in series with source 3l to the suppressor gridof pentode I4.

In a somewhat similar manner, diodes 23 to 26 are caused to conduct bythe E. M. F. of the secondary of transformer I8. In this case, sources38 to 4I are so connected that, initially, all anodes are negative withrespect to their catliodes, and no current traverses them until acertain E. M. F. is attained. The voltages of the various sources ofpotential are so selected that initial conductivity takes place in diode24 or 25 before diode 25 or 23 respectively. During that portion of thecycle in which that end of the secondary which is connected to diodes 24and 525 is positive, only diode 24 will be active and the resultantcurrent in resistor 33 will be such that the end nearest the transformerwill be negative. The rate of increase of this potential will beproportional to the rate of increase of the modulating E. M. F., untilthe condition is reached at which the voltage of sources lo and 4I, plusthe potential across resistor 33, is exceeded and, as the modulating E.M. F. is further increased, diode 23 will conduct, and the resultingcurrent will traverse resistor 32 or 33, or divide between them. Thetransformer secondary E. M. F. applied to diode 2t is made equal to thatapplied to diode 2li, and resistors 32 and 35 are of equal value andhigh in comparison with the resistance of the diodes in the conductingcondition. As a result, any modulating E. M. F. in excess of thatrequired to start current through diode 26 has negligible eiiect uponthe current in resistor 33, the whole of this current being equal to theadditional current passing through diode 24.

During any portion of the cycle in which the end of the secondary oftransformer iS connected to diodes 2li and 25 is negative, a similaraction will take place, but in this case diodes 23 and 25, and resistors3i and 3:3 are involved and the potential developed across the resistor3&3 is in the reverse sense. As a result, the relationship betweeninstantaneous modulating E. M. F. and potential drop across resistor 33is as illustrated in Figure 3. From O to A neither idode is active, fromA to B diode 2li conducts, and from B to C both are conducting.Referring again to Figure l, the resistor 3o is connected across aseparate secondary winding and the relation between the potential dropacross it and the modulating E'. M. F. will be a linear one, as shown inFigure 4. The slope of this graph is determined by the transformer ratioand is made exactly one-half that of the slope or portion AB of thegraph of Figure 3.

Resistors il@ and 33 are connected in series and the sum of thepotentials developed across them is applied to the suppressor grid ofpentode E5. The characteristic of this summated potential is obtained byadding together the graphs of Figures 3 and 4, obtaining the graph ofFigure 5.

The outputs of tubes i3, ill and i5 are fed to the tuned circuits 1&3,lili; lit, dl; and 49, 5o respectively and the outputs of the coils 55,dil and 5i respectively coupled thereto are fed to the network ofresistors 52 to 53, as shown in Figure l., and thence to the utilizationmeans 59.

The relation between instantaneous amplitude of R. F. output of atyp-ical pentode in terms of instantaneous modulating E. M. F. appliedto its suppressor grid is shown in Figure 6. By making the correctselection of all electrode potentials by adjusting the impedanceconnected in the anode circuit, it is readily possible to adjust thischaracteristic to have the same shape from point D to point E, to a veryclose degree of approximation, as a sine curve between the values tf andtf The voltage of source 3l and the potential drop across resistor 2aare so xed that the upper limit of potential applied to the suppressorgrid of pentode il! (points F of Figure 2) correspond with y thepotential required to produce the R. F. output at point E on Figure 6.Similarly, the lower limit (points G of Figure 2) is made to correspondwith the modulating potential required for the condition illustrated atD on Figure S. The net result is that the R. F. output characteristic oftube ifi, showing the relation between instantaneous modulating E. M. F.and radio irequency output, will be as the graph of Figure 7, i. e., toa close degree of approximation of the form of 1+ cos 0 between thelimits 0=-2t to +21r. In a similar manner, the R. F. outputcharcteristic of pentode i5 can be shown to be the combination of thegraphs of Figures 5 and 6 and is illustrated in Figure 8. Thischaracteristic has the form of the curve of 1+ sin 0 plotted between thelimits 0:-211- and 0=l2m Pentode iii is unaffected by the E. M. FXSappearing in the anode circuit of pentode i5 and vice-versa. However,the potential between points M and N in the resistance network is thehalf sum or half diierence of the E. M. F.s of coils and 5i, dependingupon the sense in which these coils are connected to the network. rIhispotential is applied to points S and R of the network consisting ofresistors 55 and 58 and to points P and Q of this network is connectedthe output or coil A5. By adjustment of the electrode potentials ofpentode I3, the E. M. F. appearing at coil i5 is made to be \/1/2 thatof the M. F. of coil 5i in the unmodulated condition. As a result, thereappears in .resistor 5l' the vector sum of three radio frequencycurrents, derived from the output circuits of pentodes i3, ill and I 5,having the relative magnitudes respectively, when no modulation isapplied, ci V, 2, I. The phase dierence between these currents is thatset up by the phasing network 2 2 and the sense in which coils de, d8and 5I are connected to the resistor network 52 to 58. These connectionsare made to produce the phase angles illustrated in Figure 9, where O-i3represents the current due to pentode i3 (relative value V55, Omi@ thatdue to pentode lli (relative value ZI), and

0*!5 that due to pentode l5 (relative value I).

The vector sum of these potentials is equal to I and is in phase withO-lll.

Let us now consider what occurs on modulation. Select any instantaneousvalue oi modulating E. M. F., as indicated by 9 in Figures 7 and 8. Thecurrent due to'pentode i e will have the value I+I cos 0. The currentdue to pentode iii will have the value I+I sin 0. The current due topentode I3 will remain at V51'. The phase angles between these currentsis the same as before modulation, but, due to the change in magnitude ofO-i and O-i5, the vector sum is changed. It has the same magnitude as before, but is shifted through the angle 0, as shown in Figure 10, exactlyin proportion to the amplitude of the modulating E. M. F. Thus thereappears across resistor 57 a potential the magnitudeI of which does notchange, but the phase of which is shifted in direct proportion to theamplitude of the modulating E. M. F. up to the limit set by the numberof diodes employed-in this case, eight-permitting a complete revolutionin each sense for full modulation. This potential is applied to theutilization means 59. Addition of more diodes (one pair for eachadditional right angle), together with appropriate transformer secondarytaps, and sources of bias potential is all that is required forextension of the permissible angle. Note that all bias sources shown areat ground potential insofar as modulating M. F.s are concerned, andhence may be constituted by power rectiers.

The expression frequency modulated current as used herein is intended toinclude a phase modulated current.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be conned to the embodimentsshown, but what I desire to cover by Letters Patent is set forth in theappended claims.

I claim:

l. The method of producing a frequency modulated current which comprisesthe steps of so modulating a first current of substantially constantfrequency in accordance with a modulating potential that itsinstantaneous amplitude is proportional to a function of the sine of acertain variable angle, so modulating a second current displaced inphase from the first current by a substantially constant angle of 90degrees in accordance with said modulating potential that itsinstantaneous amplitude is proportional to the same function of thecosine of said variable angle, causing the magnitude of said variableangle to be directly proportional to the instantaneous magnitude of saidmodulating potential, and combining the two amplitude modulated currentsto produce the frequency modulated current.

2. The method of producing a frequency modulated current which comprisesthe steps of so modulating a first current of substantially constantfrequency in accordance with a modulating potential that itsinstantaneous amplitude bears a certain ratio to the sine of a certainvariable angle plus unity, so modulating a second current displaced inphase from the flrst current by a substantially constant angle of Qdegrees in accordance with said modulating potential that itsinstantaneous amplitude bears said ratio to the cosine of said angleplus unity, causing the inagnitude of said angle to be directlyproportional to the instantaneous magnitude of said modulatingpotential, and combining the two amplitude modulated current, with athird current of constant amplitude which bears said ratio to the squareroot of two and which is displaced in phase from the first two currentsby substantially constant and equal angles of 135 degrees to produce thefrequency modulated current.

3. A device for producing a frequency modulated current comprising meansfor producing a First current of substantially constant frequency, afirst modulating means arranged to vary the instantaneous amplitude ofthe first current, means for obtaining a second current displaced inphase from the first current by a substantially constant angle of 90degrees, a second modulating means arranged to Vary the instantaneousamplitude of the second current, a rst means for converting themagnitude of a modulating potential to a value which is a function ofthe sine of an angle directly proportional to the instantaneousmagnitude of the modulating potential, a second means for independentlyconvertthe magnitude of said modulating potential to a value which is afunction of the cosine of said angle, means for applying the convertedmodulating potentials separately and simultaneously to the twomodulating means, and means for combining the two amplitude modulatedcurrents to produce the frequency modulated current.

4. A device for producing frequency modulated currents comprising meansfor producing a first current of substantially constant frequency, afirst modulating means arranged to vary the instantaneous amplitude ofthe first current and having a response characteristic upon applicationof a modulating potential thereto which is a function of the sine of anangle directly proportional to the instantaneous magnitude of thepotential, means for obtaining a second current displaced in phase fromthe first current by a substantially constant angle of 90 degrees, asecond modulating means arranged to vary the instantaneous amplitude ofthe second current and having a response characteristic upon applicationof a modulating potential thereto which is a function of the cosine ofsaid angle, means for applying a modulating potential simultaneously tothe two modulating means, and means for combining the two amplitudemodulated currents to produce the frequency modulated ciu'- rents.

5. A device for producing frequency modulated currents comprising meansfor producing a rst current of substantially constant frequency, meansfor obtaining a second current displaced in phase from the first currentby a substantially constant angle of degrees, means for obtaining athird current displaced in phase from the first two currents bySubstantially equal angles of '135 degrees, separate modulating meansarranged to vary the instantaneous amplitudes of the first and secondcurrents and having response characteristics upon application of amodulating potential thereto which are functions of the sines ofcomplementary angles, both angles being linearly related to theinstantaneous magnitude of the moduiatinaY potential. means ff applyinga modulating potential simultaneously to the two inoduatng means, andmeans for combining the .-,fo amplitude modulated currents with thethird unmodulated current to produce the frequency modulated current.

6. A device as in claim 5 in which said separate modulating meansincludes means having response characteristics whch are functions of thesines of the espec'tive angles plus unity.

7. A device for producing frequency modulated currents comprising meansfor producing a rst current of substantially constant frequency, meansfor obtaining a second current displaced in phase from the first currentby a substantially constant angle of 90 degrees, a first modulatingmeans arranged to vary the instantaneous amplitude of the rst currentand having a response characterstp with respect to a modulatingpotential, which characteristic, when plotted graphically, has a shapewhich is substantially that of a sinusoidal wave between two successiveand opposite peaks, a first network arranged to modify a modulatingpotential so that it cyclically and linearly increases and decreaseswith increasing magnitude of the modulating potential connected to applythe modified modulating potential to the rt modulating means, a secondmodulating means similar to the first modulating means connected to varythe instantaneous amplitude of the second current, a second networksimilar to the rst network except having cyclic peaks of the modiedmodulating potential disposed intermediate cyclice peaks of the firstnetwork with reference to the magnitude of the modulating potentialconnected to apply the modified modulating potential to the secondmodulating means, means for applying the modulating potentialsimultaneously to both networks, and means for combining the twoamplitude modulated currents to produce the frequency iodulated current.

8. A device as in claim 7 in which each of the module ing meanscomprises a thermionic tube having a control grid and a suppressor gridin which the current whose amplitude is varied is applied to the controlgrid and the modified modulating potential is applied to the suppressorgrid.

9. A device as in claim 7 in which each network comprises a plurality ofbiased rectiers connected to produce the cyclic and linear modificationof the modulating potential.

10. A device as in claim 7 in which the combining means comprises agroup of resistive circuit elements connected as a balanced bridge, thetwo amplitude modulated currents being applied to points thereof whereone current will have no appreciable eiect upon the other.

1l. A device for producing frequency modulated currents comprising meansfor producing a rst current of substantially constant frequency, meansfor obtaining a second current displaced in phase from the first currentby asubstantially constant angle of 90 degrees, means for obtaining athird current displaced in phase from the rst two currents bysubstantially equal angles of 135 degrees, a first modulating meansarranged to vary the instantaneous amplitude of the rst current andhaving a response characteristic with respect to a modulating potentialwhich characteristic, when plotted graphically, has a shape which issubstantially that of a sinusoidal wave between two successive andopposite peaks, a first network arranged to modify a modulatingpotential so that it cyclically and linearly increases and decreaseswith increasing magnitude of the modulating potential connected to applythe modified modulating potential to the rst modulating means, a secondmodulating means similar to the iirst modulating means connected to varythe instantaneous amplitude of the second current, a second networksimilar to the iirst network except having cyclic peaks of the modifiedmodulating potential disposed intermediate cyclic peaks of the firstnetwork with reference to the magnitude of the modulating potentialconnected to apply the modified modulating potential to the secondmodulating means, means for applying the modulating potentialsimultaneously to both networks, and means for combining the twoamplitude modulated currents with a predetermined constant portion ofthe third current to produce the frequency modulated current.

12. A device as in claim 11 in which each of the modulating meanscomprises a thermionic tube having a control grid and a suppressor gridin which the current whose amplitude is varied is applied to the controlgrid and the modified modulating potential is applied to the suppressorgrid.

13. A device as in claim 1'1 in which each network comprises a pluralityof biased rectifiers connected to produce the cyclic and linearmodication of the modulating potential.

14. A device as in claim 11 in which the combining means comprises twogroups of resistive circuit elements, a rst group being connected as abalanced bridge, the rst group being interconnected with the circuitelements of the other group to form a further balanced bridge in whichthe first group constitutes one arm thereof and the two amplitudemodulated currents and the third current are applied to points of thetwo balanced bridges such that no one of the three currents will have anappreciable effect upon either the other two.

LIONEL T. BIRD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,130,172 Armstrong Sept. 12,1938 2,210,968 Wirkler Aug. '13, 1940 2,220,201 Bliss Nov. 5, 19402,347,458 Brown Apr. 25, 1944

